1. Field of the Invention
The present invention relates to an amplitude detecting method for detecting the amplitude of an alternating current signal, and an AGC circuit gain controlling method for controlling the gain of an AGC circuit by using the amplitude detecting method.
In addition, the present invention relates to an amplitude detecting apparatus for detecting the amplitude of an alternating current signal, and an AGC amplifier circuit which includes an amplitude detecting apparatus as a component, and which controls the gain of an AGC circuit.
2. Prior Art
An AGC amplifier circuit according to the prior art has a function of outputting an alternating current signal having a constant amplitude in response to fluctuations in the amplitude of an inputted transmission signal (alternating current signal). In order to implement this function, the AGC amplifier circuit has an AGC circuit and an amplitude detecting circuit for detecting the amplitude of an output signal of the AGC circuit, and is provided with an automatic gain controlling (AGC) mechanism where an amplitude detection signal of the amplitude detecting circuit is fed back to the AGC circuit.
In the above described configuration, when the DC voltage, which is the amplitude detection signal of the amplitude detecting circuit, is not constant, but rather fluctuates, the gain of the AGC circuit also fluctuates, and the waveform of the transmission signal that is outputted from the AGC circuit is distorted, because the time constant of the feedback loop is significantly small.
Next, FIG. 8 shows the configuration of an AGC amplifier circuit according to the prior art using a smoothing condenser. This AGC amplifier circuit is formed of an input terminal 1, an AGC circuit 2, an output terminal 3 and an amplitude detecting circuit 10.
AGC circuit 2 amplifies a transmission signal a that has been inputted into input terminal 1 by the gain in accordance with the gain control signal and outputs the resulting signal, and thereby, the amplitude of output signal b is made constant, irrespectively of the fluctuation in the amplitude of transmission signal a.
Amplitude detecting circuit 10 that is used to detect the amplitude of output signal b of AGC circuit 2 is formed of an input terminal 4 of the amplitude detecting circuit, an output terminal 5 of the amplitude detecting circuit, a full wave rectifier 11 and a smoothing condenser 12.
Thus, in this configuration, full wave rectifying is carried out on output signal b of AGC circuit 2, and in addition, the signal is smoothed, and thereby, an amplitude detection signal c1 which corresponds to the amplitude of output signal b is outputted. This amplification detection signal c1 is supplied to AGC circuit 2 as a gain control signal.
FIG. 9 shows operation waves in each portion of the AGC amplifier circuit of FIG. 8. FIG. 9 shows the waveform of transmission signal a, the waveform of output signal b of AGC circuit 2, the waveform of amplitude detection signal c1 of amplitude detecting circuit 10 and a change in gain G of AGC circuit 2, respectively. Here, in the waveform of amplitude detection signal c1, the broken line indicates a case where there is no smoothing condenser, and the solid line indicates a case where there is a smoothing condenser.
When smoothing condenser 12 is not utilized, the time constant of the feedback loop formed of AGC circuit 2 and amplitude detecting circuit 10 is small. Therefore, the wavelength of amplitude detection signal c1 becomes the same as the waveform of the output of the full wave rectifier, as shown by the broken line. As a result of this, gain G of AGC circuit 1 fluctuates a great deal, and thereby, the waveform of transmission signal b that is outputted from AGC circuit 2 is distorted.
In contrast to this, when smoothing condenser 12 is used, the time constant of the above described feedback loop increases, and the fluctuation in the waveform of amplitude detection signal c1 decreases, as shown by the solid line. As a result of this, the fluctuation in gain G of AGC circuit 1 is reduced, and thereby, the distortion in the waveform of transmission signal b that is outputted from AGC circuit 1 is reduced.
Patent Document 1: Japanese Unexamined Patent Publication H08 (1996)-172330
In the configuration of FIG. 8 according to the prior art, smoothing condenser 12 is used in order to improve the waveform so as to reduce distortion, and therefore, a problem arises, where the time constant of the feedback loop is increased, and the speed of response of the AGC amplifier is lowered.